Particulate solids are introduced into well formations for a variety of purposes. In hydraulic fracturing operations, particulate proppants are carried into fractures created in the subterranean rock formation by hydraulic pressure. Proppants suspended in a fracturing fluid are carried into the fractures and upon releasing the fracture pressure, the proppants remain in the fractures holding the separated rock formation apart to create channels for the flow of formation fluids, e.g., hydrocarbons including natural gas and oil, back to the well bore and ultimately to the well head.
It also is common to place particulate material in the area surrounding a well bore to maintain permeability and control sand entrainment. Such gravel packs, as they are called, act as filters to restrict the flow of fines and formation sand with the hydrocarbon fluid into the well bore. Typically, gravel or sand having a mesh size between 10 and 60 mesh on the U.S. Standard Sieve Series is placed in the region adjacent to the well bore; these particles may be bonded together using a thermosetting resin composition.
Notwithstanding these techniques and often as a consequence of them, particulate solids are generated during the operation of a well that are sufficiently buoyant to be transported by the formation fluid (hydrocarbon) as part of the recovery effort. For example, the nature of the formation itself may be populated with particles sufficiently small to be entrained in the formation fluid. When these transported particulates remain in the formation fluid recovered at the well head, premature wearing of the hydrocarbon production equipment becomes a problem. Such particulates also can clog the well bore significantly reducing, if not halting, the well's production rate. Eventually, the solids must be removed from the fluid adding additional cost to the recovery operation.
Proppant flowback is one example of this phenomenon in which the proppant itself is dislodged from the fracture and becomes entrained in the formation fluid (hydrocarbon) as it is recovered from the well. As noted above, the entrained solids can cause undue wear on the production equipment and in severe cases can also reduce formation conductivity.
The longstanding nature of this problem has engendered a wide variety of potential solutions.
One of the most common approaches to reduce proppant flowback has been to employ thermoset (cured) resin-coated or thermosetting (curable) resin-coated proppants. Typical resins include epoxy resins and phenol-formaldehyde resins. In this approach, exemplified for example in U.S. Pat. No. 4,336,842, U.S. Pat. No. 5,128,390 and U.S. Pat. No. 5,639,806, the resin-coated proppant is introduced into the formation. In the case of the curable resin-coated proppants, the pressure encountered in the formation fractures causes the thermosetting resin-coated proppant to agglomerate or bridge one-to-another and the attendant heat causes the resin to cure-in-place. Upon curing, the consolidated nature of the agglomerated proppants fix the material in-place.
U.S. Pat. No. 4,869,960 describes using a cured novolac epoxy resin for coating the proppant.
Another approach is described in U.S. Pat. Nos. 5,330,005; 5,439,055 and 5,501,275 where fibers are added into the formation in hopes that they form a mat or framework structure that helps to hold particulates in place and reduce flowback.
U.S. Pat. No. 5,501,274 describes adding a thermoplastic material, such as a polyolefin, polyamide, polyvinyl or cellulose derivative, in individual particulate, ribbon or flake form along with the proppant in an amount of 0.01% to 15% by weight of the proppant. Once the proppant and separate elements of the thermoplastic material lodge in the formation, softening of the thermoplastic material occurs causing bridging between proppant particles and the separate particles of the thermoplastic material, leading to the formation of agglomerates. These agglomerates hopefully create a framework structure in the formation, much like the cured-in-place thermosetting resin coated proppants, retarding flowback from the formation. According to U.S. Pat. No. 5,582,249 the thermoplastic material can be coated with an adhesive. According to U.S. Pat. No. 5,697,440, the thermoplastic material may also be an elastomeric material, also in individual particulate, ribbon or flake form is added with the proppant. As above, the elastomeric material preferably softens at the temperature encountered in the formation so that the elastomeric particulates, ribbons or flakes adhere to the proppant.
In U.S. Pat. Nos. 5,330,005, 5,439,055 and 5,501,275 a fibrous material is added to the treating fluid having suspended therein the particulate solids (e.g., proppant) and the treating fluid is introduced into the subterranean formation. It is suggested that the fibers act to bridge across constrictions and orifices in the proppant pack. The bridging forms a mat or framework that holds particulates in place and limits flowback.
U.S. Pat. Nos. 5,775,425, 5,787,986, 5,833,000, 5,839,510, 5,853,048, 6,047,772 and 6,209,643 use a tackifying compound for coating at least a portion of the particulates introduced into a formation. The tackifying compound causes particulates adjacent the coated material to agglomerate and create a framework structure in the formation. The '510 patent also includes a hardenable resin in the formulation so that curing of the resin then acts to fix that agglomerated structure in-place. The tackifying compound is a liquid or a solution that partially coats the particulate substrate prior to or subsequent to placement of the particulate in the formation. The tackifying compound forms part of the treatment fluid suspension for delivering the particulates into the formation. Specific examples of a tackifying compound include polyamides and liquids and solutions of polyesters, polycarbamates, polycarbonates and natural resins such as shellac. A main drawback of this method is that the coating of the tackifier must be done at the well site or the tackifier must be transported to the well as a slurry. Once the tackifier is applied to the proppant, the proppant is no longer free-flowing.
In U.S. Pat. No. 6,832,650, reticulated foam fragments are mixed into the treating fluid along with the particulate material (proppant) as a way of reducing or preventing the flow-back of solids into the recovered fluid.
Notwithstanding these various approaches, the interest in developing new solutions to the problem of particulate generation and transport in well recovery operations remains strong. Choosing the correct proppant remains an important aspect of successful well stimulation and recovery operations.